Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths. When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme, although semiconductor optical amplifiers are also used in some situations. At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link is shared by all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine the channels into, and separate the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
Minimally, the optical signal generators, e.g., the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength control. Especially in systems with center-to-center wavelength channel spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any wander impairs the demodulation of the wandering signal at the far end receiver since the wavelength is now at a wavelength different than expected by the corresponding optical signal detector, and the wandering signal can impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
In addition to wavelength stability, optical signal generators that are tunable are also desirable for a number of reasons. First, from the standpoint of manufacturing, a single system can function as the generator for any of the multiple channel wavelength slots, rather than requiring different, channel slot-specific systems to be designed, manufactured, and inventoried for each of the hundreds of wavelength slots in a given WDM system. From the standpoint of the operator, it would be desirable to have the ability to receive some wavelength assignment, then have a generator produce the optical signal carrier signal into that channel assignment on-the-fly. Finally, in higher functionality systems such as wavelength add/drop devices, wavelength tunability is critical to facilitate dynamic wavelength routing, for example.
Tunable laser systems have been proposed in which a gain media, such as a semiconductor optical amplifier (SOA) chip, is installed in a laser cavity along with a Fabry-Perot tunable filter or cavity. These systems address the issue of preventing off-resonant light, i.e., light rejected (reflected) by the Fabry-Perot tunable filter, from re-entering the SOA chip and being amplified. Specifically, these proposed systems place a discrete polarizer filter between the Fabry-Perot filter and the SOA chip along with a Faraday rotator. Essentially, the polarizer filter blocks FP-rejected light from entering the chip since it has been rotated 90 degrees by the rotator. The problems exist, however, with these systems insofar as they are complex and costly to implement.
According to the present invention, an SOA chip is selected that is polarization anisotropic. As a result, it generates substantial light at only one polarization. This polarization is based on the strain, i.e., compressive or tensile, characteristics of the quantum well and thus generates either TE or TM polarized light. As a result, because of this configuration, it will not amplify light at the orthogonal polarization. Thus, by selection of such an SOA chip, the rejected light can be simply rotated to be orthogonal to the light generated by the SOA chip. This light will propagate through the SOA chip and not be amplified. Further, because of the SOA chip""s relatively low reflectivity front facet or equivalent front reflector, most of this off-resonant light will be dissipated out the front of the laser cavity, and thus, the SOA will not amplify the rejected light.
In general, according to one aspect, the invention features a semiconductor tunable laser system. It comprises a front reflector and a back reflector defining a laser cavity. A semiconductor laser chip is installed within this cavity that is polarization anisotropic and thus only amplifies light at an amplification polarization. A filter is further provided in this laser cavity that has a tunable passband for selecting a wavelength of operation of the laser system. A first polarization rotator is located between the semiconductor chip and the tunable filter. This first rotator that rotates the polarization that is emitted from the semiconductor chip and is outside of a passband of the tunable filter such that a polarization of the rejected light is substantially orthogonal to the amplification polarization upon returning to the semiconductor chip. Further, a second polarization rotator is utilized in the laser cavity that rotates a polarization of light that is emitted from the semiconductor chip and is within the passband of the tunable filter such that the polarization of the pass light is substantially parallel to the amplification polarization upon returning to the semiconductor chip.
Depending on the implementation, various systems can be used for the first polarization rotator and the second polarization rotator. In one embodiment, Faraday rotators are used. In other embodiments, since the polarization of the light emitted from the SOA chip is known and invariant due to the polarization anisotropy, quarter-wave plates or sub-wavelength gratings are used for the first and second polarization rotators.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.